1. Field of the Invention
This invention relates to the monitoring of optical signal quality in optical networks and, more particularly, to an all-optical system and method for measuring timing jitters of ultra-high-bit-rate optical signals.
2. Background of the Related Art
The monitoring of signal quality in ultra-fast optical networks is a crucial aspect of optical networking. Two major contributors to the bit-error-rate are timing jitter and amplitude jitter. As data transmission rates increase to 40 GHz and beyond, the detection of both jitters by electronic means becomes very complex and costly. Moreover, common jitter measurements using the so-called eye-diagram are flawed, because the experimental results do not necessarily support the conclusion that the signal degradation is due to either timing jitter, amplitude jitter, or a combination of both. In general, additional assumptions based on the knowledge of the jitters' origin are required to allow the evaluation of both types of jitter.
Recent advances in the areas of low-jitter laser sources and high-bit-rate optical networks have boosted the network bit rate to 40 Gbps and beyond. At these higher bit rates, timing jitter is one of the major factors affecting system performance. Timing jitter increases the bit-error-rate in long-haul data transmission systems. Thus, there is a need for time-resolved jitter monitoring and measurement techniques.
Traditional power spectrum analysis techniques for measuring timing jitter, such as those discussed in Von Der Linde., “Characterization of the noise in continuously operating mode-locked lasers”, Appl. Phys. B., Vol. B39, No. 4, pp. 201-17 (1986), are not always suitable for applications involving ultra-high repetition rates. This is, in part, because of the so-called “electronic barrier”, i.e., the necessary electronics may not be readily available or lacks precision as the data rate increases. In the GHz range, only a few jitter experiments have been reported on harmonically mode-locked fiber lasers. See, for example, T. F. Carruthers et al., “10-GHz, 1.3-ps erbium fiber laser employing soliton pulse shortening”, Opt. Lett., Vol. 21, No. 23, pp. 1927-9 (1996). In T. R. Clark et al., “Sub-10 femtosecond timing jitter of a 10-GHz harmonically mode-locked fiber laser”, OFC/IOOC 1999, PD24/1-3, a measurement of a 10 GHz fiber laser is described that achieved a precision exceeding 10 fs by employing a doubly balanced mixer to shift the RF spectrum of interest down to low frequencies. Nevertheless, a considerable amount of high-speed electronics is required, including a high frequency reference electronic or electrical signal from the source, such as that described in G. Sucha et al., “A new method for rapid temporal scanning of ultrafast lasers”, IEEE J. Sel. Top. Quantum Electron., Vol. 2, No. 3, pp. 605-21 (1996). Today's ultra-high-bit-rate optical networks require a timing-jitter monitoring system with temporal resolutions down to fractions of the intrinsic temporal pulse width itself.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.